1. Field of the Invention
The present invention relates to a defect pixel detection apparatus and a method for detecting a defect pixel. More specifically, the present invention relates to a technique for detecting a defect pixel of an image sensor in high accuracy.
2. Description of the Related Art
Concerning a solid-state image sensor used in an image sensing apparatus, e.g., charge coupled device (CCD) type image sensors and complementary metal oxide semiconductor (CMOS) type image sensors, it is known that, if there is a defect in a pixel, noise such as a dark current is generated in an output signal due to the defect of the pixel. If such phenomenon occurs in an optical black pixel section (OB section) or a dummy pixel section (Null section), the phenomenon adversely effects a clamp operation of each of the OB section and the Null section and causes a horizontal or vertical streak on an image.
The OB section is an area that includes a first pixel which includes a photoelectric conversion element such as a photodiode and a layer for outputting a signal generated by the photoelectric conversion element of which surface is optically shielded by, for example, aluminum. The Null section includes no photoelectric conversion element, or is an area including a second pixel that does not have the layer for outputting a signal generated by the photoelectric conversion element including the photoelectric conversion element.
The above described dark current fluctuates significantly according to a usage environment (e.g., an ambient temperature, exposure, or the like). Therefore, to obtain a high-quality image, a highly accurate detection of defect pixel and correction of thus detected defect pixel are required even when the usage environment changes. Japanese Patent Laid-open Publication No. 2004-222143 discusses a method for detecting a defect pixel. In this method, a threshold for determining defect in each pixel is set for every photographing condition, e.g., a photographing mode, a shutter speed, and sensitivity in order to accurately detect the defect in the pixel which changes according to the usage environment.
Japanese Patent Laid-open Publication No. 2001-268448 discusses another method for detecting a defect pixel. In this method, there are two detection units for an effective pixel section and the OB section, respectively, and a different detection threshold is set for each of the detection units. Each of the detection units calculates a temperature and exposure time, and an alarm is issued according to a detected defect level of temperature and exposure time.
The method discussed in the Japanese Patent Laid-open Publication No. 2004-222143, however, can not deal with an increase of a dark current caused by change of the ambient temperature. In other words, a detection threshold suitable for each defect level which varies according to the usage environment cannot be appropriately set. Therefore, the defect pixel cannot be detected in high accuracy, and the defect pixel may be erroneously corrected. As a result, a horizontal or vertical streak may be generated on an image after correction.
Further, this method requires that the OB section and the effective pixel section behave in a similar manner with respect to the change of the usage environment. However, as it is described above, the OB section, the Null section, and the effective pixel section have physically different pixel configurations. Therefore, a temperature dependency of the defect level is not always the same between those sections. As a result, the defect in the pixel cannot be detected in high accuracy in each of the sections and thus an erroneous correction may occur in each section if the usage environment changes.
In the method discussed in the Japanese Patent Laid-open Publication No. 2001-268448, a determination whether to accept an element is performed at a peak value of the defect level using the set threshold. Therefore, if there is even a single defect of a high level in the pixel, the element will be rejected. In other words, there is a problem that even the image sensor of a good quality may not be accepted due to a single defect in the OB section. Since the Null section has no threshold different from that of the OB section, there is another problem that the erroneous correction may be made due to a difference in pixel configuration.